If you buy a screen that was formerly iCloud locked, it won’t make no difference to the screen. Only difference is the device itself it WAS iCloud locked. So yes, you can take a LCD screen off an iCloud-locked device, since it only effects the logic board INSIDE the phone, all the other parts won’t be effected. So everything else can be tested like it should, though i’m not sure if Face ID will work as it is from a difference phone, but it is worth trying if that’s what your curious about.